1. Field of the Invention
The present invention relates to displacement detecting devices that detect relative displacement between an objective lens and a surface to be measured by using an optical astigmatic method. More specifically, the present invention relates to a displacement detecting device having a light adjustment member disposed between a light source and an objective lens so as to adjust the resolution of output light focused on a surface to be measured or having a light adjustment member disposed between an objective lens and a light receiving element for regulating an incident angle of reflected light reflected by the surface to be measured so as to accurately measure the surface roughness of the surface to be measured.
2. Description of the Related Art
In the related art, displacement detecting devices have been widely used for measuring the displacement and the shape of a surface to be measured (which will be referred to as a “measurement surface” hereinafter). In a displacement detecting device, a laser beam emitted from a light source of a non-contact sensor is focused on a measurement surface through an objective lens, and a focus error signal is generated by an astigmatic method on the basis of reflected light reflected by the measurement surface. By performing servo control using this focus error signal, the focal length of the objective lens is shifted for adjustment. By reading the divisions on a linear scale integrally attached to the objective lens by means of a link member, displacement of the measurement surface is detected.
However, the above-described displacement detecting device is problematic in that it is difficult to achieve high detection accuracy since the focus error signal itself has poor linearity. Japanese Unexamined Patent Application Publication No. 5-89480 proposes a displacement detecting device that is configured to output a corrected output signal corresponding to the focus error signal of the non-contact sensor from a correction table. In this displacement detecting device, the beam diameter of light to be focused on an object to be measured is reduced by increasing the numerical aperture of the objective lens in order to achieve higher accuracy in the displacement detection. For example, the displacement detecting device uses a laser beam with about 2 μm in beam diameter (wavelength) as output light and achieves detection accuracy of about several nanometers to a hundred and some nanometers on the linear scale.